In the Homilies of Saint Jerome (a 4th-5th Century theologian) it is demonstrated that in his times, Sabbath observance (Saturday) was wholly expected.
Homily 21 on Psalm 92 we read,
“The Ninety-first Psalm is inscribe with the title: ‘A psalm; a song for the Sabbath day.’ There could be no Sabbath day without six preceding days. We work for six days, on the seventh day we rest. We cannot sing to the Lord, therefore, save on the day of the Sabbath. As long as we are engaged in the works of the world, that is, for the six days, we cannot sing to the Lord. Leviticus says: ‘On the Sabbath day you shall do no servile work.’ No one, therefore on the day of the Sabbath and on the day of the Lord’s rest may do servile work—work pertaining to this world; but he ought to do the work that belongs to the Sabbath. Would you know that on the Sabbath the priests work in the temple of the Lord? It is not permitted anyone to cut wood on the Sabbath; in fact, the man who was discovered gathering wood in the wilderness was stoned to death. Neither may one even kindle a fire nor do any kind of work.
You will observe, however, that all the things that the layman is not allowed to do on the Sabbath, the priests alone are permitted to do, for they cut wood, enkindle a fire, and perform other services, and immolate victims. Why am I making such a point of this? To show you that it is written in the law that we must withdraw from all worldly pursuits on the Sabbath and perform only those works that pertain to God. A psalm, therefore, is a song on the Sabbath day when we do not work for the world, but for God. Let us see now what we must sing on the Sabbath when we are abstaining from the works of the world.
……. [He then breaks down verse by verse, the entire Psalm 92.]
From Homilies of Saint Jerome, Volume 1 (1-59 on the Psalms)
Also around 404 A.D. St. Jerome wrote: “…the believing Jews do well in observing the precepts of the law, i.e. …keeping the Jewish Sabbath…there exists a sect among (…) the synagogues of the East, which is called the sect of the Minei, and is even now condemned by the Pharisees. The adherents to this sect are known commonly as Nazarenes; they believe in Christ the Son of God, born of the Virgin Mary; and they say that He who suffered under Pontius Pilate and rose again, is the same as the one in whom we believe” (Jerome. Translated by J.G. Cunningham, M.A. From Jerome to Augustine (A.D. 404); LETTER 75 (AUGUSTINE) OR 112 (JEROME). Excerpted from Nicene and Post-Nicene Fathers, Series One, Volume 1. Edited by Philip Schaff, D.D., LL.D. American Edition, 1887. Online Edition Copyright © 2004 by K. Knight).
Contact us at firstname.lastname@example.org to pre-order a comprehensive booklet on how the Sabbath has been, and continues to upheld in the Orthodox Church of God. Online version available here: “The 7th Day Sabbath in the Orthodox Church“.